Why is the link between the Caxton Hill industrial estate and the Foxholes industrial estate closed off?

It always has been. Both estates are privately owned and I think there were probably issues with the link being used as a shortcut between Ware Road and the A414 - avoiding the Mill Road lights and Bluecoates roundabout.

Closed off not only by concrete blocks but also invariably great piles of fly tipped s**t

That flytip has been there for over a year - I reported it when it first appeared, both to the council and site owner. Clearly the owner can't be bothered.